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Cerebellar transcranial magnetic stimulation impairs verbal working memory.

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Cerebellar Transcranial Magnetic Stimulation
Impairs Verbal Working Memory
John E. Desmond, PhD,1 S. H. Annabel Chen, PhD,2 and Perry B. Shieh, MD, PhD3
Previous functional magnetic resonance imaging and patient studies indicate cerebellar participation in verbal working
memory. In particular, event-related functional magnetic resonance imaging showed superior cerebellar activation during
the initial encoding phase of the Sternberg task. This study used functional magnetic resonance imaging–guided transcranial magnetic stimulation (TMS) to test whether disruption of the right superior cerebellum (hemispheric lobule
VI/Crus I) impairs verbal working memory performance. Single-pulse TMS was administered immediately after letter
presentation during the encoding phase on half the trials. Sham TMS and a Motor Control task were included to test for
general distraction and nonmemory-related motor effects. Results showed no effects of TMS on accuracy, but reaction
times (RTs) on correct trials were significantly increased on TMS relative to non-TMS trials for the Verbal Working
Memory and Motor Control tasks. Additional analyses showed that the increased RT was significantly greater for Verbal
Working Memory than for the motor task, suggesting that the effect on working memory was not caused by interference
with finger responses. Sham TMS did not affect RTs, indicating that the potentially distracting effects of the postencoding click did not contribute to the increase in RT. The observed effects from cerebellar disruption are consistent with
proposed cerebrocerebellar involvement in verbal working memory.
Ann Neurol 2005;58:553–560
Neuroimaging techniques have demonstrated a surprisingly consistent presence of cerebellar activation during
a variety of cognitive functions1 including attention,2
language,3–7 explicit memory retrieval,8 –12 and verbal
working memory.13–18 These findings are generally
consistent with studies of cerebellar-damaged patients
that have shown cognitive deficits such as perseveration
and impaired verbal fluency,19 impaired planning,20,21
agrammatism,22 associative learning deficits,23–26 and
impaired verbal working memory.27,28
Although patient studies have been important for establishing cognitive functions of the cerebellum, these
investigations have four major shortcomings. First,
damage may not be completely restricted to the cerebellum,29 raising the possibility that brainstem damage
may contribute to the observed deficits. Second, although patient studies can indicate whether disruption
of a structure affects performance, they cannot demonstrate at what point in time during the task that the
structure makes its contribution. Third, although it is
implicitly assumed that the damaged region is the
source of impaired behavioral performance and would
have shown evidence of contributing to the task
through changes in activation, it is not possible to
verify these changes in nonreversibly lesioned patients.
Finally, brain reorganization and compensatory mechanisms in patients may obscure the original contributions of the damaged structure.
The potential effects of reorganization and compensation is illustrated in a case report by Silveri and colleagues28 of a patient who underwent surgical removal
of a mass in the right cerebellar hemisphere. This patient exhibited significant and specific deficits in verbal
working memory shortly after surgery that were ascribed to deficits in articulatory control. The deficits
were resolved after 5 months, at which time the patient’s performance was completely normal. This result
suggests that cerebellar cognitive deficits in patient
studies may be obscured by compensation from other
brain regions, depending on the length of time that has
elapsed since surgery.
Transcranial magnetic stimulation (TMS), a brain
stimulation technique that involves passing a powerful
and rapidly changing current through a coil that is
placed on the scalp, has the potential for demonstrating cognitive cerebellar contributions in the absence of
such compensation.30 –32 TMS is an ideal complement
to functional magnetic resonance imaging (fMRI) because it can test whether disruption of fMRI-identified
regions of activation produces changes in performance,
From the 1Department of Neurology, Johns Hopkins University,
Baltimore, MD; 2Department of Psychology, National Taiwan University, Taipei, Taiwan; and 3Department of Neurology, University
of California, Los Angeles, School of Medicine, Los Angeles, CA.
Published online Sep 26 2005 in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/ana.20604
Received Jan 25, 2005, and in revised form Jun 28. Accepted for
publication Jun 29, 2005.
Address correspondence to Dr Desmond, 1620 McElderry Street,
Reed Hall East-2, Baltimore, MD 21205.
E-mail: dr.jdesmond@gmail.com
© 2005 American Neurological Association
Published by Wiley-Liss, Inc., through Wiley Subscription Services
553
thereby distinguishing which regions are necessary for
task performance from those that are simply correlated
with performance.
Thus, this study used single-pulse TMS to investigate whether cerebellar disruption affects cognitive performance. Specifically, we tested whether cerebellar
TMS would impair performance of the Sternberg Verbal Working Memory task. Although the contribution
of neocortical structures to Verbal Working Memory
has been explored with TMS, similar investigations
have not been performed for the cerebellum. As illustrated in Figure 1, this task can be broken down into
three distinct temporal phases of encoding, maintenance, and retrieval. Several neuroimaging studies have
reported reliable cerebellar activation during performance of this task,13–18,33,34 and more recent eventrelated fMRI investigations have shown that activation
in the right superior cerebellar hemisphere (lobule VI,
Crus I) occurs in response to the encoding phase of the
task.35,36 Thus, for this initial investigation of cerebellar involvement in verbal working memory, we targeted
(using fMRI) the right superior cerebellar activation for
this task and administered TMS immediately after
stimulus encoding.
Subjects and Methods
Subjects
Seventeen healthy, right-handed male (n ⫽ 8) and female
(n ⫽ 9) subjects gave their informed consent to participate
in this study, which was approved by an Institutional Review
Board and a US Food and Drug Administration Investigational Device Exemption. All subjects were screened for
TMS contraindications and received neurological testing
(tandem walking, finger-to-nose test, and heel-knee-shin
tests) before and after TMS. There were no adverse consequences or complaints from any of the subjects that required
discontinuation of the experiment. The mean age of the subjects was 24.9 ⫾ 6.9 years (mean ⫾ standard deviation).
Stimuli
Stimuli consisted of uppercase and lowercase English consonants generated from a Macintosh computer (Apple Computer, Cupertino, CA) using PsyScope software.37 For TMS
laboratory experiments, subjects viewed stimuli directly from
a Macintosh notebook screen. For fMRI experiments, the
stimuli were visually presented to the subject by backprojecting the images, via a magnet-compatible projector,
onto a screen located above the subject’s neck. The images
were viewed from a mirror mounted above the subject’s
head.
Procedure
The experimental methods for this study were accomplished
in three sequential steps, with each step typically occurring
on a separate day. For the first step, subjects received fMRI
scans while performing two tasks: a finger tapping task with
the left hand to identify motor cortex, and a Verbal Working
Memory task to identify the cerebellar target for TMS administration. They also received a high-quality anatomical
MRI scan for creating a brain surface rendering. For the sec-
Fig 1. Tasks used for transcranial magnetic stimulation (TMS) experiments. (A) The Verbal Working Memory task consisted of a
1.5-second visual presentation of six letters followed by a maintenance period of 4 seconds for covert rehearsal, followed by a retrieval period consisting of a four-letter probe presentation. Subjects were required to press a button indicating which probe letter
had been presented during the encoding period. TMS administration occurred immediately after the encoding period on 50% of the
trials. (B) The Verbal Working Memory (Sham Coil) task was the same task as described above, but the TMS coil delivered sham
stimulation designed to replicate the sound of a genuine TMS trial but did not stimulate the brain. (C) The Motor Control task
consisted of presentation of visual stimuli during the encoding period (# symbols) that did not have to be subsequently remembered.
As in the Verbal Working Memory task, TMS occurred after the encoding period in 50% of the trials. For the retrieval period of
the task, the subject was required to press a button indicating which probe letter was an x.
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ond step, fMRI activations from the two scans were mapped
onto separate brain surface renderings, using Brainsight Frameless (Rogue Research, Montreal, Quebec, Canada; http://
www.rogue-research.com) software. Four scalp landmarks,
located on the left and right ears (superior edge of the tragus), the tip of the nose, and the bridge of the nose, were
also identified on the MRI scans. For the third step, the four
scalp landmarks were coregistered with the corresponding
landmarks on the MRI scan using the Brainsight Frameless
system. The methods for this system have been described in
detail elsewhere38 – 40 and have been used in previous TMS
investigations.41,42 The transformation resulting from coregistration was used to interactively guide the center of the
TMS coil to the desired brain region. TMS stimulation of
the motor cortex was used to obtain each subject’s finger
twitch threshold, and once this was accomplished, cerebellar
stimulation was targeted for the three tasks depicted in Figure 1.
Transcranial Magnetic Stimulation Administration
Single-pulse TMS was administered using a Magstim 200
stimulator and 110mm double cone and sham double cone
coils manufactured by Magstim (Carmarthenshire, United
Kingdom; www.magstim.com). The sham double cone coil
emitted a click similar to that of the genuine coil on stimulation, but it did not stimulate the brain. The sham coil was
used to assess the distracting effect of the TMS click on performance. For genuine TMS administration to the cerebellum, the coil was oriented so that current was directed
downward (ie, inferiorly), as has been used successfully in
other studies of cerebellar stimulation with this coil.43 Cerebellar TMS magnitude was set to 120% of motor cortex
threshold, but in three cases, a lower magnitude was used at
the request of the subject. In these cases, the cerebellar stimulation magnitudes were 88%, 106%, and 114%. Motor
cortex threshold was determined by visual inspection of left
hand and finger twitching on stimulation of right motor cortex. For motor cortex stimulation, the double cone coil was
oriented so that current was directed posteriorly, roughly
perpendicular to the central sulcus.
Tasks
FUNCTIONAL MAGNETIC RESONANCE IMAGING LOCALIZATION. The Verbal Working Memory task used for lo-
calizing cerebellar activation was based on the Sternberg paradigm,44 and it has been used successfully in previous studies
to characterize cerebellar involvement in this cognitive function.14,36,45 In this task, 6 uppercase consonants (High Load)
or 1 uppercase consonant and 5 “#” symbols (Low Load)
were presented for 1.5 seconds. (For the Low-Load stimuli,
the position of the letter in the array varied randomly across
all six possible positions.) The stimuli were then erased from
the screen, and the subjects were required to keep the letters
in mind during a 5-second maintenance period using covert
rehearsal. After the maintenance period, a probe stimulus,
consisting of one lowercase consonant letter, was presented
for 1 second. For the High-Load condition, this probe
matched one of the six letters in the array on half the trials.
For the Low-Load condition, the probe matched the one
non-“#” letter on half the trials. Subjects were instructed to
press the button under their index finger when the probe
letter matched an array letter and to press the button under
their third (middle) finger if it did not. A period of 1.5 seconds after the offset of the probe letter was allowed for responses; thus, each trial was 9.0 seconds in duration. Four
trials were presented in each 36-second block. Six cycles (12
blocks) of High and Low Load were presented for a total of
432 seconds.
To identify the right motor cortex, we instructed subjects
to perform a finger tapping task. When the word Go appeared (in green letters), subjects were required to tap two
buttons under the index and middle fingers of the left hand
until the word Stop (in red letters) appeared. Go and Stop
blocks were each 30 seconds in duration, and 5 cycles of the
task were performed for a total of 5 minutes.
Figure
1 illustrates the three tasks that were performed by subjects
for the TMS experiment. For each task, subjects received 96
trials, 1 every 8.8 seconds. Half the trials (“Stim” trials) included TMS (or sham TMS) administration. The other half
was “No Stim” trials; that is, no TMS or sham TMS administration occurred. The order of the trials was pseudorandomized, and no more than three consecutive Stim or No
Stim trials were allowed. As indicated in Figure 1, the time
of TMS administration on Stim trials was immediately after
offset of the 1.5-second encoding period. For each Working
Memory trial, six letters were always visually presented during the encoding period. After a 4-second maintenance period for covert rehearsal, 4 lowercase probe letters were presented, and the subject was required to press 1 of 4 buttons
(oriented in 1 linear row) to indicate which probe letter had
appeared in the initial encoding period. All possible combinations of target letter position (six possible) and correct
probe letter position (four possible) were presented. A Motor
Control task (see Fig 1C) was included to assess the effect of
TMS on finger reaction time (RT) performance in the absence of working memory. For this task, subjects were required to press the button corresponding to the position of
the letter x, which occurred with equal frequency in all four
probe positions. The order of task performance was counterbalanced across subjects. Stimulus lists used for the genuine
TMS and sham TMS Verbal Working Memory tasks were
also counterbalanced.
TRANSCRANIAL MAGNETIC STIMULATION TASKS.
Data Acquisition and Analysis
Imaging was performed with a 1.5Twhole-body MRI scanner (General Electric Medical Systems Signa, Waukesha, WI)
using a custom-built quadrature “top hat” birdcage head coil.
Structural MRI protocols consisted of a SpinEcho localizer
scan and a T2-weighted Fast SpinEcho anatomical scan
(coronal acquisition: TE ⫽ 90msec; TR ⫽ 3,000msec; field
of view ⫽ 24cm; 256 ⫻ 192 matrix; number of excitations
(NEX) ⫽ 1.0; slice thickness ⫽ 6mm; 1 interleave; 30 slices)
and a T1-weighted Spoiled Grass (SPGR) image with the
following parameters: TR ⫽ 9msec; TE ⫽ 1.9msec; flip angle ⫽ 15 degrees; 124 slices; field of view ⫽ 24cm; 256 ⫻
192 matrix; reconstructed resolution ⫽ 0.9 ⫻ 0.9 ⫻
1.5mm. Whole-brain fMRI data were acquired with a T2*weighted gradient-echo spiral pulse sequence (TE ⫽ 40msec;
Desmond et al: Cerebellar TMS Impairs Memory
555
TR ⫽ 3,000msec; flip angle ⫽ 89 degrees; inplane resolution ⫽ 3.75mm; slice thickness ⫽ 6mm with 0mm gap; 30
coronal slices; field of view ⫽ 24cm; 1 NEX).
Image preprocessing and statistical analyses were performed using the Statistical Parametric Mapping 99
(SPM99) software package (Wellcome Department of Cognitive Neurology, London, UK). The functional images
were subjected to motion correction, and then coregistered
to the SPGR scan. Statistical preprocessing consisted of
high-pass filtering at 144 seconds and low-pass filtering
through convolution with the SPM99 canonical hemodynamic response function. An SPM{t} map was then created
by contrasting the High- and Low-Load working memory
conditions. Surface renderings of the SPGR scan together
with coregistered functional data were performed using
Brainsight Frameless software. Examples of cerebellar renderings from five subjects, which include color-coded functional activations for targeting TMS administration, are illustrated in Figure 2.
Results
Behavioral Performance
A repeated measures analysis of variance was performed separately for percentage accuracy and median
RT data for accurate trials, with factors of condition
(Working Memory, Working Memory with Sham
Coil, and Motor Control) and trial type (Stim and
No Stim). For accuracy data, subjects correctly identified the probe letter in 71.3 ⫾ 13.0% of the Stim
trials (mean ⫾ standard deviation) and 69.9 ⫾
10.1% of the No Stim trials for the Verbal Working
Memory task using the genuine TMS coil. Similar
performance was observed for the Sham TMS coil,
with 69.8 ⫾ 12.6% accuracy for Stim trials and
70.3 ⫾ 12.7% accuracy for No Stim trials. Greater
accuracy was observed in the Motor Control task,
with 96.3 ⫾ 2.7% accuracy for Stim trials and
98.2 ⫾ 2.2% accuracy for No Stim trials. Analysis of
variance showed a significant main effect of condition
(F[2,32] ⫽ 76.83; p ⬍ 0.0001) but not trial type
(F[1,16] ⫽ 0.21; p value was not significant), and
there was no significant interaction (F[2,32] ⫽ 1.075;
p value was not significant). Thus, subjects were more
accurate for the Motor Control task than for the
Working Memory task, but there was no difference in
accuracy between Stim and No Stim trials.
In contrast with the accuracy data, there was a significant condition x trial type interaction for RT on
correct trials, as illustrated in Figure 3. For the Verbal
Working Memory task with the genuine TMS coil,
subjects responded to the probe with a RT of
1,250.5 ⫾ 403.1msec (mean ⫾ standard deviation) on
Stim trials and 1,134.8 ⫾ 315.6msec on No Stim trials. With the sham coil, the Stim and No Stim RTs
were 1,191.4 ⫾ 339.6msec and 1,160.9 ⫾ 330.2msec,
respectively. For the Motor Control task, Stim and No
Stim RTs were 667.4 ⫾ 111.3msec and 654.3 ⫾
101.5msec, respectively. Analysis of variance showed
significant effects of condition (F[2,32] ⫽ 44.0; p ⬍
0.0001), trial type (F[1,16] ⫽ 9.7; p ⬍ 0.007), and
condition x trial type (F[2,32] ⫽ 3.445; p ⬍ 0.044).
Subsequent tests of Stim versus No Stim RTs indicated
that TMS administration to the cerebellum significantly increased RT for both the Motor Control task
(t[16] ⫽ 2.44; p ⬍ 0.027) and Verbal Working Memory task (t[16] ⫽ 2.58; p ⬍ 0.02). However, sham
Fig 2. Subject-specific localizations of right superior cerebellar activation obtained from the Verbal Working Memory task. Surface
renderings of the cerebellum used for transcranial magnetic stimulation (TMS) targeting of lobule HVI/Crus I are depicted for five
subjects. Activations represent a contrast of the High-Load versus the Low-Load conditions.
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Fig 3. Reaction time (RT) results for Stim and No Stim trials
for each of the three task conditions. Genuine transcranial
magnetic stimulation (TMS), but not sham TMS, significantly
increased RT during the retrieval period of the task.
TMS had no significant effects on RT (t[16] ⫽ 1.26; p
value was not significant). Further analysis of the condition x trial type interaction (Fig 4) indicated that the
RT difference between Stim and No Stim trials for the
Working Memory task was significantly greater than
the RT difference for the Motor Control task (t[16] ⫽
2.34; p ⬍ 0.032).
Discussion
The results of this study indicate that right superior
cerebellar TMS administered immediately after letter
presentation increased response time to the probe but
did not affect accuracy. The increase in RT from cerebellar TMS for the Verbal Working Memory task
significantly exceeded the RT increase for the Motor
Control task, which had similar response requirements but no memory requirements. This suggests
that the TMS RT effect on working memory was not
simply caused by interference with finger motor responses. However, that a significant motor response
was observed is further evidence that TMS stimulation affected the desired superior cerebellar structures.
Results from the sham TMS coil showed no significant difference between Stim and No Stim trials. This
indicates that, to the extent the Sham coil click accurately reproduces the general distracting effects of
TMS (that include both auditory and somatosensory
components), the increase in RT was also not simply
due to distracted performance.
The change in verbal working memory performance
produced by single-pulse TMS is consistent with neuroimaging evidence, suggesting a role for superior cerebellum in verbal working memory. These data have
shown that the right superior cerebellum, together with
premotor cortex and Broca’s area, exhibit activation in
response to the encoding phase of the Sternberg
task.35,36,46
In accordance with this hypothesized role in rehearsal, previous investigations have shown that both
superior cerebellar and inferior frontal activation can be
elicited not only by the Sternberg task, but also by a
nonmemory control task requiring repeated encoding
and covert rehearsal of visually presented letters. These
results suggest that the superior cerebellar and inferior
frontal/premotor activations do not represent phonological storage of verbal items per se, but rather the
articulatory component of the phonological loop.14,45
This may partially explain why RT, but not accuracy,
was affected in this experiment. The functional neuroimaging evidence is consistent with numerous clinical
observations of articulatory deficits occurring after
damage to Broca’s area,47,48 to the superior cerebellum,28,49 or to medial pontine regions50 that link frontal and superior cerebellar regions.51,52
In summary, a convergence of neuroanatomical,
functional neuroimaging, and cerebellar patient studies
suggest that the right superior cerebellum is involved in
articulatory preparation processes that likely rely on
connections with left inferior frontal and premotor regions. Activation of the superior cerebellum in speech
production appears to be more readily observed when
faster articulatory preparation is required.53 Such rapid
preparation is prominent in the Sternberg Verbal
Working Memory task during the 1.5-second encoding
period, especially at higher loads, where rapid and efficient planning of the articulatory trajectory is needed
for the ensuing maintenance period. The results of this
TMS study are therefore consistent with the hypothesis
that the superior cerebellum computes an articulatory
trajectory immediately after letter presentation that facilitates the phonological loop. Cerebellar TMS, appropriately timed at a critical point in the trajectory computation, is hypothesized to interfere with this
facilitation. The compromised phonological loop results in slower extraction of the items during the subsequent retrieval period, and consequently slower responses to the probe.
Fig 4. Reaction time (RT) increases for Stim relative to No
Stim trials. Significantly greater RT increases were observed
for the Verbal Working Memory task than for the Motor
Control task.
Desmond et al: Cerebellar TMS Impairs Memory
557
Although this explanation for our TMS results is
consistent with evidence from neuroimaging and patient studies, other explanations are possible. For example, Theoret and colleagues54 demonstrated increased variability in timing accuracy after repetitive
cerebellar TMS. Given the hypothesized role of the
cerebellum in the representation of temporal information,55,56 it is possible that RT increases in our study
reflect TMS disruption of a timing component required for the somewhat rhythmic sequential repetition of consonant sequences during the rehearsal process.
Previous studies have investigated the effects of
TMS administration to neocortical regions on verbal
working memory performance. For the parietal cortex, negative results for recall of digit sequences were
obtained using 50Hz trains of stimulation to electroencephalogram electrode locations P3 and P4,57
which likely correspond to Brodmann Area (BA) 7.58
However, single-pulse TMS to the posterior third of
the intraparietal sulcus bilaterally resulted in increased
errors on a two-back task.41 Herwig and colleagues,59
using a Sternberg task, found greater errors for left
relative to right BA 7/40 stimulation that approached
significance with a small sample size (N ⫽ 6), and
Grafman and colleagues60 observed impaired recall of
words occurring at the end of the list using 20Hz
trains of TMS to this region, but only on the left
side. These researchers also observed impaired recall
for words near the beginning of the list when TMS
was applied to temporal regions (T5,6 electrodes, corresponding bilaterally to BA 37). For the frontal cortex, Feredoes and colleagues61 reported negative results for stimulation in the vicinity of BA 8/9 for a
three-back task, and Herwig and colleagues reported
negative results with single-pulse stimulation to BA 9
for a Sternberg task.59 However, Mull and Seyal62
observed significant disruption using single-pulse
stimulation to BA 46 in a three-back task, Grafman
and colleagues60 observed it with stimulation trains to
left or right BA 45/46 for serial recall of words, and
Mottaghy and colleagues41,63 also observed significant
disruption with either single-pulse or train stimulation to left or right BA 46/9 in a two-back task. Left
premotor cortex stimulation (BA 6) was found to be
more effective than dorsolateral prefrontal cortical
stimulation for impairing performance on the Sternberg task.59
The results of our study extend the findings of
these previous studies by demonstrating cerebellar
participation in verbal working memory performance.
The observed impairment of verbal working memory
performance from cerebellar disruption is consistent
with the anatomically based cerebrocerebellar verbal
working memory model that Desmond and colleagues14 originally described, which predicted, based
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on neuroanatomical patterns of corticocerebellar connectivity, concordant activations of superior cerebellar
with frontal regions and concordant activations of inferior cerebellum with parietal regions during verbal
working memory. These predictions have been confirmed in recent investigations of task-phase–dependent activation,36 conjunction analyses of working
memory versus articulatory activation,45 and loadand practice-related cerebrocerebellar activation.64
In summary, the results of this study suggest that
TMS can be used effectively for investigating cerebellar contributions to cognitive function. The temporal
specificity of this transient lesion technique can provide complementary data to support and extend the
findings from patient studies and neuroimaging investigations.
This work was supported by the NIH (National Institute of Mental
Health, MH60234, J.D.).
We gratefully acknowledge J. Boshart for subject recruitment and
project coordination.
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